Present earphones generally reduce the influence of environmental noise on human ear using Active Noise Reduction (ANR) technology. ANR technology usually comprises Feed Forward ANR circuit (FF ANR) or Feed Back ANR circuit (FB ANR), or comprises both.
Implementation of FF ANR usually need place a Reference Microphone (REF MIC) outside an earphone (the earphone is positioned outside the auditory meatus when worn) for perceiving environmental noise. The REF MIC signal is played via a Speaker (SPK) after being processed by earphone inner circuit and the signal played offsets the environmental noise that is transmitted to the external auditory meatus to eliminate the influence of environmental noise on human ear. Implementation of FB ANR usually need place an Error Microphone (ERR MIC) inside an earphone (the earphone is positioned inside the auditory meatus when worn) for perceiving environmental noise that penetrates the earphone. The ERR MIC signal is played via the Speaker after being processed by earphone inner circuit and the signal played offsets the environmental noise that is transmitted to the external auditory meatus to eliminate the environmental noise.
FIG. 1 is a structure diagram of an ANR earphone. FIG. 1 shows a REF MIC 101 placed outside the earphone, an ERR MIC 102 placed inside the earphone and a Speaker 103.
According to the technology adopted by ANR earphones, ANR earphones can be classified into Feed Forward Active Noise Reduction (FF ANR) earphone, Feed Back Active Noise Reduction (FB ANR) earphone and Hybrid Active Noise Reduction (Hybrid ANR) earphone.
FIG. 2A is a functional block diagram of a FF ANR earphone. FIG. 2B is a functional block diagram of a FB ANR earphone. FIG. 2C is a functional block diagram of a Hybrid ANR earphone. In FIG. 2A and FIG. 2C, FF ANR module performs corresponding processing on signals collected by a REF MIC and displays them via a Speaker (SPK); in FIGS. 2A, 2B and 2C, OUTPUT denotes earphone outputting signal, such as musical signal that is played, voice from the other side of the phone, and the like. Environmental noise signal is picked up by a REF MIC and an ERR MIC and is played via the SPK after being processed by the FF ANR module and the FB ANR module. The voice signal played by the SPK is again picked up by the REF MIC and the ERR MIC, and again played via the SPK after being processed by the FF ANR module and the FB ANR module respectively. Positive feedback will be formed when some condition is satisfied, and thus a howling is produced.
FIG. 3 is a modeling diagram of a howling. Open-loop response is defined as TO(z, n)=G(z)F(z, n). Wherein z denotes frequency point and n denotes time. The condition of producing howling is, at some frequency fOsc, satisfying<TO(2πfOsc,n)=k2π,kεN ═TO(2πfOsc,n)|>1
then the feedback system is unstable, creating vibration, and thus the howling is produced. When aforesaid condition is satisfied, amplitude of signal of which frequency is fOsc increases exponentially in the cyclic process of G(z)→F(z, n)→G(z), and the amplitude tends to be infinite after repeatedly circulating in ideal state. However, for the ANR earphone, it usually increases till reaching the maximal amplitude value owning to the limitation of total voltage of the circuit or MIC amplitude.
FIG. 4 is a modeling diagram of a howling of a FF ANR earphone. As is shown in FIG. 4, the forward direction path transfer function of the system is TFREF˜SPK; the feedback path transfer function is TFSPK˜REF; When howling condition is satisfied, a howling is produced.
FIG. 5 is a modeling diagram of a howling of a FB ANR earphone. As is shown in FIG. 5, the forward direction path transfer function of system is TFERR˜SPK; the feedback path transfer function is TFSPK˜ERR; When howling condition is satisfied, a howling is produced.
For the Hybrid ANR earphone, when feed forward loop or feedback loop satisfies the howling condition, or feed forward and feedback loop simultaneously satisfy the howling condition, or functions of feed forward and feedback loop combine together to satisfy the howling condition, then a howling is produced.
After the howling is produced, power of the Speaker playing reaches the maximum; sound pressure level at MIC reaches the highest; and electric current on circuit reaches the maximum, thus it is likely to damage the Speaker and MIC and power consumption will increase prominently, and the circuit is likely to be burnt out. After the howling, the Speaker will emit sound wave of high sound pressure level at the frequency point of howling, which is likely to cause discomfort to users.
Function of the howling suppression is suppressing howling to avoid damaging components and circuit or causing discomfort to users. The howling suppression generally comprises two parts: howling detection and howling processing. Howling detection is to detect whether or not a howling is produced at present or whether or not a howling is likely to be produced at present; howling processing is to break the positive feedback loop that causes howling production, so that a howling is not produced. The howling processing method of the ANR earphone comprises amending ANR parameters or shutting down ANR circuit, etc.
The feature of a howling is that the howling is usually produced at some frequency point, while environmental noise, voice, music and the like are usually broadband signals. Therefore, howling suppression method usually adopted by prior arts performs detection by using the feature of frequency-domain of a signal of a howling, i.e. monofrequency signal detection method. Detecting a monofrequency signal is considered as a howling is produced, and then howling processing should be performed to suppress howling. Specific procedure is first converting the digital signal that is converted by A/D to frequency-domain, and dividing the frequency-domain into several different frequency bands and detecting which frequency band has howling via the method of peak-to-average ratio of the frequency-domain, and then performing frequency suppression on the frequency band with a howling. This practice can be used for Feed Forward, Feed Back and Hybrid ANR earphones. However, the weakness of the practice is that the howling can only be detected after the howling is produced, that is, there is a short period of howling time. If the practice is applied to ANR earphones, a transitory howling might appear. That is, users can hear a short howling, and the MIC and SPK might be damaged since the howling is produced. Thus the best method is to avoid the production of a howling.